1. Field of the Invention
This invention concerns perforating guns and, more particularly, linear charge perforators used to perforate subsurface formations penetrated by oil and gas wells and the like.
2. Discussion of Prior Art
Generally, in completing oil and gas wells, a metal casing is positioned in a well bore. A cementitious material is then usually pumped into the annular space between the metal casing and the well bore. The cement permanently fixes the metal casing in the well bore and prevents migration of well fluids through the annular space. The casing, cement and formation are then perforated by either shaped charges or bullet perforators at the level of a producing formation.
However, channels often form in the cement behind the casing and it is desirable to close off those channels to prevent migration in the well of unwanted fluids through the cavities in the cement located space or annulus between the well bore and the casing.
The present system for perforating water channels and the cement behind the casing uses standard point charges at various angles of phasing to create a helix of shots in the casing wall. This system has used as low as 15.degree. phasing which, when using four jet shots per foot, provides a 360.degree. turn every six feet for a total of twenty-four shots. An example of such perforating apparatus is seen in U.S. Pat. No. 3,415,321, entitled "Shaped Charge Perforating Apparatus and Method," issued to A. A. Venghiattis Dec. 10, 1968.
As can be recognized in illustrations FIGS. 6 and 7 of that patent, a water channel in the cement behind casing might be missed using such perforating apparatus. Consequently, it is advantageous to provide a device which will insure close to 100% reliability of actually perforating a water channel in the cement behind the casing.